This invention relates to sampled data systems, and, more particularly, to digital conference systems for providing intercommunication between selected stations.
One of the major problems with prior art intercommunications systems is that they are primarily designed to communicate between only a single pair of stations. Typically, these intercom systems have a very limited conferencing capability wherein a single conference can be maintained between a very limited number of stations within the system. Once this single conference is in progress, no other conferences can be conducted until the conferencing circuitry is relinquished by the first group.
A few prior art intercom systems are designed to handle conferences. These systems are typically analog rather than digital designs. One type of prior art analog conference system requires that a very large number of cables be routed from each remote station. The main element of this system is a sophisticated rotary switch. This system has a number of disadvantages in addition to its extreme complexity and high cost. First, it is very easy to monitor so that an unauthorized party could gain access to sensitive conversations. Second, there are problems of crosstalk between various users involved in separate conferences. Furthermore, adapting a voice scrambler circuit to this prior art conferencing system to eliminate security risks is a complex and costly undertaking. In addition, since this is an analog system it requires periodic adjustment and alignment and it cannot be fabricated by using large scale integrated circuit techniques.
Other forms of prior art conference systems require highly sophisticated intercom stations, each one of which serves essentially as a decentralized data processor to separate out the desired incoming data and properly encode the outgoing data. These remote intercom stations are highly complicated and the possibility of a failure or breakdown is thereby increased.
Another disadvantage of prior art conferencing systems is that because of the size and complexity of the intercom stations and because of the great amount of cabling between the intercom stations and the central switching station, the weight of the entire system is substantial. For airborne applications where weight is a highly significant factor and where the space required for complex cabling arrangements is extremely limited, use of this type of system is virtually precluded.
An additional drawback of the prior art conferencing systems which is particularly acute in airborne installations is the lack of a braodcast mode wherein someone (such as the pilot) can broadcast emergency instructions or other high priority information to every intercom station simultaneously. This feature is particularly necessary for airborne installations, but is also highly desirable in other ground-based installations. An additional feature which many prior art conferencing systems have not been able to conveniently provide is the ability to have a single listener simultaneously monitor all other system conversations. Furthermore, it is highly desirable to have a conferencing system which can call up preprogrammed nets so that certain operators can enter into frequently used conference networks without having to individually dial up each station with which he wishes to confer.